Turbomachinery devices extract energy from moving fluids (air, combustion gases, water, steam, etc.) or impart energy to those fluids. Under certain conditions, solid material can deposit from the fluids moving through the turbomachinery. At the very least, these deposits, generally referred to as fouling, compromise efficiency by roughening aerodynamic surfaces. In the extreme, fouling deposits can grow to fill internal passages, throttling flow.
The precise nature of the deposits fouling a device will vary with the composition of the gas flow. For example, organic polymers can deposit within units handling hydrocarbon gases, while inorganic crusts form within turbomachinery operating on wet steam. In any case, efficiency is lost through three basic mechanisms:
Increased friction against gas flow, reduction of flow path cross-sectional area, and random changes of pressure distribution on airfoil.
Polymerization is one type of fouling that plagues centrifugal compressors pumping hydrocarbons in the process chemical and petrochemical industries. These chemical compressors deliver gases at volumes, pressures and temperatures critical to large scale chemical processes.
A centrifugal compressor acts on gas by means of a bladed impeller. The rotating impeller imparts a centrifugal force on the process gas resulting in an increase of both its tangential and radial velocity. The velocity's tangential component is then converted into an increase in pressure in the diffuser passage of the diaphragm. The important individual aerodynamic components of a multi-state centrifugal compressor include the inlet nozzle, inlet guide vanes, impeller, radial diffuser, return channel, collector volute and discharge nozzle.
The inlet nozzle accelerates the gas stream into the guide vanes which distribute the flow evenly to the first stage impeller. The rotating impeller forces the gas into the diffuser formed by stationary components called diaphragms. The diffuser reduces gas velocity and converts kinetic energy in the stream into higher pressures. Because gas flows through the diffuser in a spiral manner, it needs to be straightened before it enters the next impeller stage. This is done by the use of return channel vanes which are also part of the diaphragm assembly. It should also be noted that due to the pressure rise that is generated, the diaphragm is a structural, as well as an aerodynamic component. The collector volute and discharge nozzle reduce gas velocity prior to discharge.
Polymerization fouling of aerodynamic hardware in centrifugal compressors pumping hydrocarbons reduces operating efficiencies and component reliability. Though this problem has been known for years, relatively little work has been done to understand it and devise ways to minimize its impact. The most probable reason for its lack of attention is that fouling alone usually does not lead to a catastrophic machine malfunction or unscheduled downtime and can generally be handled by shortened maintenance intervals, where components are cleaned by removing accumulated polymer, returned to near original operating condition, and put back into service. These operations cause loss of efficiency.
Polymerization is not well understood, as it applies to compressor fouling. What is known is that the hydrocarbons which are inherent to the process gas or formed during the compression process can bond tenaciously to the component base metals and lead to significant performance loss of the machine. Deposits of this type have been found in compressors used for hydrocarbon processing, coke gas blowers, and other units where the gas contains sufficient amounts of hydrocarbons under conditions of high pressure and temperature.
Factors found to be critical to polymerization/fouling are: temperature--polymerization usually occurs above about 194.degree. F. (90.degree. C.), pressure--the extent of fouling is proportional to pressure level, surface finish--the smoother the surface the less apt the component is to foul, and gas composition--fouling is proportional to concentration of reactable hydrocarbon in the process (inlet gas).
In general, fouling has many detrimental effects on centrifugal compressors. One is build-up of material on the rotor. This build-up can lead to an unbalance which gradually builds until the unit exceeds its allowable vibration limit and has to be shutdown. Operating with significant rotor unbalance can also lead to fatigue and a reduction in component life. Fouling has also been known to reduce axial and radial clearances between the rotor and stationary components which leads to abrasive wear that severely damages impellers and labyrinth seals.
These types of fouling degradation are typical of a progressive formation of a deposit. Costs associated with correcting these problems usually show up after relatively long periods of operation.
However, polymerization fouling can occur so rapidly that efficiency losses occur very quickly, sometimes only months after start-up. In these cases, the most intense growth of deposit occurs during the first 50-200 hours of operation. The deposits affect stationary flow path components, as well as rotating elements. Prior attempts to correct the problem were limited to diffuser and return channels, stationary flow path components, which were considered to be the most susceptible to fouling. It was assumed the rotating element would be less likely to foul due to the dynamic force applied to the deposits by rotation. In addition, by design, the stationary flow paths exhibit slightly rougher surface finishes than the rotating element which make them more susceptible to deposit build-up.
A different sort of fouling afflicts steam turbines. These devices extract work from steam supplied from an external source (boiler or process vessel). Steam, superheated under very high pressures, enters the turbine and undergoes a controlled expansion as it passes over moveable and stationary airfoils (blades and vanes). The force of the expanding stream causes the blades to rotate, doing work. As the steam expands, its temperature and pressure decrease until, in later stages of the turbine, conditions are such that it spontaneously condenses on gas path surfaces.
The boundary between conditions of pressure and temperature in the steam turbine that allow condensation and those that do not is called the Wilson Line. Hardware operating well below the Wilson Line temperature and pressure will be wet. Components well above this temperature and pressure will remain dry. Since the Wilson Line will move within the turbine as operating conditions and condition of the inlet steam vary, some hardware will be alternately wetted and dried in service.
When condensation occurs, impurities carried by or dissolved within the steam precipitate and deposit on the metal surfaces. The rough hard crust which forms compromises operating efficiency in the same manner that polymerization deposits compromise centrifugal compressors. Frictional resistance to gas flow is increased, cross-sectional area available for flow is reduced, and variable pressure distributions are introduced in the unit.
In addition to loss of efficiency, fouling in steam turbines also increases the risk of corrosion damage of components. Condensing steam is an electrolyte that can initiate galvanic corrosion on a surface. This condition is exacerbated by sulfur and heavy metals, dissolved in incoming steam, which precipitate as hygroscopic salts on airfoils. When operating conditions change and steam again condenses on these dried salts, extreme pH conditions (highly acidic for deposits of acid salts, highly basic for basic salts) results immediately adjacent to the deposits. Severe corrosion, including pitting type corrosion, can result. Such corrosion not only roughens airfoil surfaces, but also compromises the mechanical integrity of the component.
Corrosion fouling is not limited to steam turbines. A similar type of fouling can occur in centrifugal compressors when there is significant moisture in the process gas stream. Water that condenses from that gas collects on compressor components, leading to aqueous corrosion.
Some attempts have been made to address and solve these industrial problems. To limit loss of efficiency due to fouling turbomachinery, various coatings have been formulated to coat gas path surfaces. It is desired that such a coating should have at least seven characteristics as follows: be smooth, non-stick, unreactive, non-wetting, thin (less than 250 .mu.m or 0.010 inches thick), adherent, and be stable up to 260.degree. C. (500.degree. F.).
Several organic coating resins, including fluorocarbons such as polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), and perfluoroalkoxy resins (PFA), or polyphenylene sulfides (PPS), are known to form smooth, unreactive, non-stick, non-wetting, thin films. However, coatings of these polymers cannot be used on turbomachinery because the non-stick materials do not remain bonded to iron, steel and nickel alloy turbomachinery components in service. Additionally, several of these fluorocarbons require a very high curing temperature, about 700.degree. F. (371.degree. C.) for PPS and PTFE, which may adversely affect the mechanical properties of the blades of the turbomachinery. Adhesion can be improved by priming the metal with a mixture of resin particles in an epoxy or other resin with a greater affinity for steels. Better yet, a primer combining polymer particles in a chromic acid slurry can be applied before the resin film. However, such coatings have not proven to be satisfactory as no such coating system adheres completely, and moisture permeating the film corrodes the metal surface below.
Aluminum-filled inorganic phosphate overlay coatings have been used to combat corrosion and erosion of steel components in turbomachinery for over 25 years. The basic coating of this type has been described in U.S. Pat. No 3,248,251 (Allen). These coatings are complex water-based slurries containing aluminum powder or alloy pigment particles dispersed in an acidic solution containing phosphates and hexavalent chromium ions which, upon exposure to heat and curing, transform to an insoluble metal/ceramic composite. Chromates (or dichromates), molybdates, vanadates, tungstates and other ions may be present. Commercial examples of such a material include Alseal.RTM. 500 and 518 manufactured by Coatings for Industry (Souderton, Pa.), CT33 manufactured by Corrotherm Inc. (Croydon, Pa.), and SermeTel W.RTM. and 962 manufactured by Sermatech International Inc. (Limerick, Pa.). These materials continue to be used in a wide variety of aerospace, automotive and industrial applications. Coating compositions of this type containing hexavalent chromium and phosphate are described in U.S. Pat. Nos. 3,248,249; 3,248,250; 3,395,027; 3,869,293; 4,537,632; 4,544,408; 4,548,646; 4,606,967; 4,617,056; 4,650,699; 4,659,613; 4,683,157; 4,724,172; 4,806,161; 4,863,516; 4,889,558; 4,975,330; 5,066,540; 4,319,924 and 4,381,323, each of which (including the patent to Allen) are incorporated herein by reference.
After such a slurry has been applied to a metal surface, usually by conventional air-atomized spraying, it is heated to a temperature between 500 to 1000.degree. F., preferably to about 343.degree. C. (650.degree. F.) until cured. At these temperatures, the phosphates and any other modifying ions, such as dichromates, undergo a series of chemical reactions to produce an inorganic amorphous glass matrix between the aluminum pigmentation and between the coating and the substrate. Once cured, the structure is frequently referred to as a "ceramic", is water insoluble, and is tightly bonded and very adherent. Tensile bond strengths of an aluminum-filled chromate/phosphate on carbon steel typically exceed 55 MPa (8,000 psi).
Each aluminum particle in the coating is discretely separate from its neighbors. This discrete separation, visible in the photograph, makes this coating a poor barrier and, though it contains about between 60-80%, or preferably about 70%, by weight aluminum, this coating is not electrically conductive.
Sacrificial or "galvanic" electrically conductive coatings prevent corrosion by corroding in the place of the substrate. When a metal that reacts more quickly in a particular environment (a more "active" metal) is placed into contact with one that reacts more slowly (a more "noble" metal), the active metal will be entirely consumed by the environment before the more noble material beings to corrode. A more galvanically active metal will corrode to protect a less galvanically active metal it when placed in a saline environment. The more active metal is said to "sacrifice" itself for the more noble one.
A number of engineering coating systems are built around the sacrificial principle. Galvanizing and zinc plating, for example, use layers of active zinc to sacrificially protect steels. Even if the sacrificial zinc layer is damaged, the active metal in the coating around the exposed substrate corrodes, halting corrosion of that substrate metal.
Inorganic phosphates that are filled with pure aluminum powder are galvanically sacrificial when they are electrically conductive. Such coatings can be made electrically conductive by thermal or mechanical post treatments. Heating the aluminum/phosphate glass composite to about 538.degree. C. (1000.degree. F.) causes the aluminum pigment and glass to react and form a semiconductor, AlP and the coating layer becomes conductive. Electrical resistivity of the coating heated in this manner will drop to less than 15 ohms when measured with probes 1" apart. The same conductivity can be achieved by lightly blasting the coated surface with abrasive grit or glass beads. And when the aluminum-filled inorganic phosphate is electrically conductive, it is also galvanically sacrificial.
For some years it has been known to apply polymer films directly on sacrificial aluminum inorganic phosphate primers. However, in service, the life of these systems on turbomachinery has always been compromised by spontaneous delamination of the polymer film from the primer.
These failures were caused by corrosion products which formed at the interface between the primer and polymer sealer. It had also been known to apply the polymer film directly onto the steel substrate material. Again, catastrophic failure occurred due to corrosion undercutting the film and delamination. During service, moisture would permeate even the best organic sealers. Corrodants dissolved in this condensate would react with sacrificial aluminum-filled basecoat. Sacrificial products (like aluminum hydroxide) would form on the primer underneath the topcoat. As these products accumulated, the topcoat tends to blister and peel from the surface. This is especially true in the high stress, high erosion environment of turbomachinery.
The parent application discloses a coating well suited to the coating of this invention which comprises a mixture of polyamide-imide, or epoxy/polyamide-imide, an ion reactive pigment and a leachable pigment, which coating corresponds to the third layer of the four layer coating of the invention.